This invention relates to a silicon carbide sintered body and a method of producing the same.
2. Related Art Statement
The silicon carbide sintered bodies have excellent chemical and physical properties, so that they are noticed as a material suitable for high temperature structures particularly used under severe conditions, such as gas turbine parts, high temperature exchanger and the like. On the other hand, the technique of producing such a silicon carbide sintered body is shifting from the conventional reaction sintering process or pressure sintering process to a pressureless sintering process wherein a mixed powder of silicon carbide, boron-containing additives and carbonaceous additives is sintered in an inert gas atmosphere under no pressure.
In the production of a so-called B-C series silicon carbide sintered body through the above pressureless sintering process, when .beta.-SiC is used as a starting material, since boron or boron-containing additive (generally B.sub.4 C), effectively used for the densification purposes added, silicon carbide grains abnormally grow to about several hundred microns at a state including bubbles in the transition from .beta. to .alpha. (abnormal grain growth) at a last sintering stage, which is different from a usual grain growth accompanying densification, so that not only the densification is suppressed but also the degradation of properties is observed. In order to solve this problem, it is necessary that the additional amount of boron is reduced, and the firing is carried out at a low temperature causing no transition from .beta. to .alpha., and the firing temperature is controlled with high accuracy.
There have heretofore been proposed various methods for reducing the additional amount of boron, for example, a method of using a specific SiC starting material which can not necessarily be obtained easily and cheaply, such as super-high purity SiC starting material containing not more than 15 ppm of metal impurity (Japanese Patent laid open No. 60-131,863); a method of conducting a pretreatment wherein a shaped body of a starting mixture is heated under a predetermined pressure and temperature and at the same time the resulting CO gas is exhausted and removed (Japanese Patent Application Publication No. 61-3,303); a method of using a pitch requiring complicated handling as a carbon source (Japanese Patent laid open Nos.60-200,861, 61-242,959 and 61-256,976) and the like. Even in these methods, however, it is required to conduct the firing at a relatively low temperature causing no transition from .beta. to .alpha. for suppressing the abnormal grain growth at the last sintering stage, so that it is difficult to achieve sufficient densification at such a low firing temperature.